The present invention relates to a process for calcining green coke obtained from a delayed coking process. More specifically, the present invention contemplates producing high-grade coke efficiently by carrying out unit process stages for calcining green coke in respectively separate heating furnaces.
Preparation of green coke from heavy oils of petroleum or coal origin such as residue oils of catalytic cracking and thermal cracking, straight run residue oils and tar of thermal cracking, coal tar pitch or mixtures thereof by a delayed coking process which comprises heat treatment at a temperature of 400.degree. to 550.degree. C. for 60 minutes to 50 hours is known. The green coke produced by this process still contains a significant quantity of moisture and volatile matter. Accordingly, there is also known a process for calcining the produced green coke in order to remove the water content and volatile matter from the green coke and to densify it, thereby producing a carbon material having a high density and a low coefficient of thermal expansion which is suitable for use as an electrode material for steel-making, aluminum smelting or the like or a carbon material for other shaped articles.
Calcining of such green coke is carried out in heating furnaces such as a rotary kiln, a rotary hearth, and a shaft kiln. That is, the raw material green coke introduced into the furnace through its inlet is dried, heated and calcined by heat of combustion resulting from the combustion of fuels, the volatile matter produced from the coke and part of the calcined coke during the time the coke is transferred to its outlet and the calcined coke is then removed from the furnace. In addition, it is well known that the calcining temperature, the rate of heating, and the furnace atmosphere in a series of calcining stages have an influence on the quality of the calcined coke. Accordingly, various types of improved processes for calcining green coke have been proposed.
One of these processes comprises pre-drying green coke in a separate apparatus by utilizing the heat of a hot gas leaving a rotary kiln before the coke is introduced into the rotary kiln (as disclosed in Japanese Patent Laid-open Publication No. 33201/1975). Another process comprises calcining green coke in a rotary kiln by supplying air through more than one opening at an intermediate part of the kiln in order to ensure complete vaporization and combustion of the volatile matter contained in the green coke which have a great influence on the quality of the calcined coke (as disclosed in Japanese Patent Laid-open Publication No. 16031/1975).
Of the above described improved processes, the former is said to be characteristic in that drying of green coke can be carried out at a low cost of operation and with good control of the process operation. However, it cannot be said that controlling of the drying process only is a substantial improvement in a calcining process for obtaining high-grade coke.
On the other hand, the latter is said to be advantageous in that the combustion of the volatile matter contained in green coke is promoted, in that the heat of combustion is utilized, and in that useless combustion of completely calcined coke is avoided. However, this process entails the following problems. A rapid temperature rise due to the combustion of the volatile matter which occurs at an air blowing place has a great influence on the quality of the resulting coke, and it is difficult to independently control the optimal temperature of the final stage of the calcining which has a great influence on the quality of the resulting coke because the calcining temperature of the final stage is greatly affected by the combustion control of the volatile matter.
Accordingly, it can be said that the above described known processes are still not fully satisfactory as processes for calcining green coke. According to the knowledge of the inventors, it is considered that the difficulties accompanying the known processes are attributable to the fact that control factors are too few as compared with the number of the unit stages included in the calcination of green coke. That is, as stated above, the calcination of green coke involves three unit stages: water removing and drying stage, volatile matter removing and combusting stage, and final calcining stage. It is preferable that these unit stages be controlled independently from each other. The reasons for this are as follows.
(1) Green coke ordinarily contains 7 to 10% by weight of water and 6 to 10% by weight of volatile matter and in the calcining process, the water is evaporated at about 100.degree. C. and the volatile matter begins to evaporate at an increased temperature of the order of 450.degree. C. That is, the respective evaporation temperatures are different from each other and the evaporated volatile matter burns and serves as a source of heat. Therefore, in order to ensure the stabilization of temperature distribution throughout the total calcining process when a raw material having different contents of water and volatile matter is used, the water removing stage and the volatile matter removing and burning stage are preferably controlled independently from each other.
(2) Green coke ordinarily contains a volatile matter content of 6 to 10% by weight or as high as 20% by weight depending upon the operation conditions of a delayed coker (the volatile matter substances are these which are defined according JIS M 8812). When this volatile matter is heated to a temperature of 450.degree. to 600.degree. C. in a heating furnace, it is evaporated, and a part thereof is melted. The melt functions as a binder forming carbonaceous adhesive matter such as ring-shaped adhesive matter (coke ring) in a rotary kiln, thereby preventing a normal flow of coke. However, if an adequate oxidizing atmosphere is maintained in the furnace, fusible volatile matter is rendered infusible in the course of temperature rise, whereby the formation of such carbonaceous materials can be prevented.
Such maintenance of an adequate oxidizing atmosphere in the volatile matter removing stage not only makes the volatile matters infusible but also improves the combustion condition thereof, which in turn affords an efficient recovery of heat. However, in the prior system wherein the volatile matter removal and the final calcining are carried out in one furnace, maintaining of a sufficiently oxidizing atmosphere so as to effectively carry out the removal and combustion of the volatile matter in the volatile matter removal stage leads to the combustion of the product coke in the final calcining stage, and this is therefore unfavorable. Thus, according to the prior system, the loss of coke is as high as about 10% by weight.
(3) Since the conditions of the final calcining stage particularly have an influence on the property of the product coke, it is preferable that the final calcining stage be controllable independently of the preceding water removing stage and volatile matter removing and burning stage.